1. Field of the Invention
The present invention relates to systems and methods for concealing the quality-degrading effects of packet loss in a speech or audio coder.
2. Background Art
In digital transmission of voice or audio signals through packet networks, the encoded voice/audio signals are typically divided into frames and then packaged into packets, where each packet may contain one or more frames of encoded voice/audio data. The packets are then transmitted over the packet networks. Sometimes some packets are lost, and sometimes some packets arrive too late to be useful, and therefore are deemed lost. Such packet loss will cause significant degradation of audio quality unless special techniques are used to conceal the effects of packet loss.
There exist prior-art packet loss concealment (PLC) methods for block-independent coders or full-band predictive coders based on extrapolation of the audio signal. Such PLC methods include the techniques described in U.S. patent application Ser. No. 11/234,291 to Chen entitled “Packet Loss Concealment for Block-Independent Speech Codecs” and U.S. patent application Ser. No. 10/183,608 to Chen entitled “Method and System for Frame Erasure Concealment for Predictive Speech Coding Based on Extrapolation of Speech Waveform.” However, the techniques described in these applications cannot be directly applied to sub-band predictive coders such as the ITU-T Recommendation G.722 wideband speech coder because there are sub-band-specific structural issues that are not addressed by those techniques. Furthermore, for each sub-band the G.722 coder uses an Adaptive Differential Pulse Code Modulation (ADPCM) predictive coder that uses sample-by-sample backward adaptation of the quantizer step size and predictor coefficients based on a gradient method, and this poses special challenges that are not addressed by prior-art PLC techniques. Therefore, there is a need for a suitable PLC method specially designed for sub-band predictive coders such as G.722.